Reversible Harmonic Drive Actuator Assembly With Low Backlash
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Solution Overview
Problem
Aircraft actuator systems face issues with flutter due to clearance in gear mechanisms, especially in rotary electrical systems, which can be catastrophic during flight, and require a reversible actuator system to ensure safety in case of motor failure, but existing systems lack the necessary backdrive capability and suffer from high backlash.
Innovation Solution
An actuator system incorporating a harmonic drive assembly with an epicyclic gear assembly and ratchet mechanism that allows disengagement in case of motor failure, enabling the actuator to return to a safe position, reducing backlash and providing a high gear ratio without the limitations of standard harmonic drives.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If a harmonic drive assembly is used to achieve high gear ratio, then the gear ratio is improved, but backlash and flutter are increased due to clearance
Solution Approach 1:
The patent applies dynamics by making the system reversible through the ratchet mechanism. The harmonic drive assembly is normally engaged to provide high gear ratio, but can be dynamically disengaged when needed. The ratchet mechanism allows the actuator to be back-driven by aerodynamic forces, creating a dynamic system that adapts its engagement state based on operational requirements, thus reducing backlash while maintaining high gear ratio capability.
Solution Approach 2:
The patent segments the torque transfer path by introducing a ratchet mechanism that can selectively engage or disengage the harmonic drive assembly from the actuator shaft. This segmentation allows independent control of the harmonic drive engagement, enabling the system to operate in engaged mode for normal operation and disengaged mode for reversal, thereby reducing backlash while maintaining high gear ratio when needed.
2Reliability
If the actuator is made reversible to allow backdrive in case of failure, then safety is improved, but clearance in the system causes undamped free movement or flutter
Solution Approach 1:
The ratchet mechanism provides dynamic reversibility, allowing the actuator to be back-driven only when necessary (e.g., motor failure). During normal operation, the harmonic drive remains engaged, maintaining tight clearance and preventing flutter. When failure occurs, the ratchet allows disengagement and backdrive, enabling safe reversal without the flutter problem present during normal operation.
Solution Approach 2:
The ratchet mechanism acts as an intermediary between the harmonic drive assembly and the actuator shaft. It mediates the torque transfer by allowing unidirectional engagement during normal operation and enabling disengagement for reversal. This intermediary component allows the system to achieve reversibility for safety while maintaining tight clearance to prevent flutter during normal operation.
3Force
If a high gear ratio mechanism is used to provide sufficient torque, then torque output is improved, but backlash increases due to clearance between gear teeth
Solution Approach 1:
The patent segments the torque transfer system into an engaged state (harmonic drive providing high gear ratio) and a disengaged state (ratchet allowing direct backdrive). This segmentation allows the system to achieve high torque output through the harmonic drive when needed while eliminating backlash by disengaging the gear mechanism during reversal operations.
Solution Approach 2:
The system dynamically switches between engaged and disengaged states of the harmonic drive assembly. During normal operation requiring high torque, the harmonic drive is engaged providing high gear ratio. During reversal operations, the ratchet mechanism enables disengagement, allowing direct backdrive without gear backlash, thus dynamically optimizing torque output while minimizing backlash.
Data Source
AI summary
An actuator system includes a motor and an actuator positioning member to be driven by the motor from a start position to an actuated position. The system also includes a harmonic drive assembly between the motor and the actuator positioning member arranged to transfer torque from the motor to the actuator positioning member at a gear ratio of greater than 1:1. The system further includes means for disconnecting the harmonic drive assembly in response to failure of the motor and/or torque transfer such as to allow the positioning member to return to the start position.